A concrete temperature stress testing machine (TSTM) may simulate the development of temperature stress and deformation of concrete under different restraint degrees and different temperature histories in a laboratory. The TSTM regulates and controls the temperature of the interior of concrete through changing the testing environment temperature, thereby controlling the development of temperature stress thereof. There are two cross-heads on both ends of concrete specimen, the cross-head at one end is a fixed cross-head and the other is a movable cross-head. When a tiny deformation of shrinkage or expansion takes place on concrete, an electric motor enables the concrete to return to the designated position by adjusting the movable cross-head, thereby the restraint conditions is achieved. The measuring deformation accuracy of the concrete specimen determines the accuracy and reliability of testing results. From the above, a temperature simulating method and a deformation measuring method of the TSTM are the technical key points that influence the performance of the TSTM.
Currently, two types of the concrete temperature control means for TSTM are mainly provided: one type is using a fluid-cooled temperature control formwork, which controls the temperature history of concrete by adjusting the temperature of the flowing fluid in the formwork, the other type is using an air-cooled environmental chamber, of which the concrete specimen is placed into the small environmental chamber, and the temperature of the concrete specimen is affected by adjusting the temperature and flowing of the air in the environmental chamber. However, at the present, both the two types of temperature control methods are only focusing on controlling the temperature near the concrete test specimen, which has some deficiencies.
A first deficiency is that, the concrete test specimen is placed in the environment of controllable temperature, however a frame and a steel shaft of the TSTM is placed in the natural environment of the laboratory, and then influenced by the environmental temperature fluctuation in the laboratory, the frame and the steel shaft of the testing machine will produce a temperature deformation, thereby influencing the deformation and strained of concrete. Taking the room temperature fluctuation of 1° C. as an example, the deformation of 12με will come about approximately concerning the steel shaft made from ordinary steel, while the threshold value controlled by the full restraint deformation of concrete is generally only 5 or 2με, the temperature deformation of the steel frame due to the environmental temperature fluctuation even exceeds the given threshold value adjusted by the concrete deformation, therefore the concrete deformation value measured by this method is not accurate.
A second deficiency is that, by the temperature control method of using the water-cooled formwork, the formwork and the concrete specimen are hard to be separated apart, the friction between the formworks and the concrete specimen will make an impact on the stress and deformation measurement during the concrete temperature stress test. In addition, a gap between the concrete formwork and the moveable cross-head should be reserved, thereby a synchronous temperature control between the concrete around the cross-head and the concrete of the middle portion contacting the formwork will not be achieved, so that a temperature gradient will be produced between the concrete around the cross-head and the concrete of the middle portion, resulting in an uneven axial temperature deformation.
A third deficiency is that, restricted by small environmental chamber, the deformation measured by the movable cross-head is utilized to present the concrete deformation. The method neglects the influence of machine rigidity and the gaps between connecting portions, so the measured deformation has much error.
A fourth deficiency is that, because the TSTM mainly consists of the steel shaft portion, the frame portion and the concrete specimen portion, the thermal expansion coefficient of each portion is different, and the temperature environment thereof is different as well, so the temperature deformation of each portion is discordant, the deformation result measured by a deformation sensor is a result containing the temperature deformation of the overall system. The temperature deformation of the system is mainly contributed by the deformation of testing machine frame and the steel shaft.
A fifth deficiency is that, concrete is an artificial material having discrete properties, in order to evaluate the material properties thereof accurately, multiple test specimens should be performed in parallel. If using the temperature simulation means of small environmental chamber or temperature formwork, it is difficult to ensure that multiple testing machines are placed in the same environment, while the synchronous control of multiple testing machines can't be achieved, thereby both the representativeness and the application universality of the testing result will be influenced.
In general, the TSTM which uses the technology of temperature control formwork or small environmental chamber can't achieve the aim that the multiple testing machines perform the test at the same time under the same temperature environment. The measured deformation can't represent the true deformation of concrete because of being influenced by rigidity of the testing machine and environment temperature. Errors of the testing results are relatively large, and even the incorrect testing phenomenon and conclusion will be obtained.